Design of high-performance high-speed electrical circuits is one of the most important issues to implement interconnections based on multilayer substrate technologies. As for an example, characteristic impedance mismatch can lead to large return losses that can dramatically degrade transferring high-speed signals at the transceiver. Multilayer substrates, which are cost-effective technologies, include planar conductive layers and vertical transitions connecting planar transmission line circuits disposed at different conductor layers. Vertical transitions in the multilayer substrate are usually realized by means of via structures.
To join a via and a planar transmission line forming an interconnect circuit in a multilayer substrate, a pad fabricated at the same conductor layer as the planar transmission line is usually used. Via pad has dimensions which are larger than via through hole diameter to provide a connection of a via and a planar transmission line. The pad has usually a circular or square form and is directly connected to the planar transmission line. However, such transition from the pad to the planar transmission line can give an increase of the characteristic impedance mismatching at higher frequencies due to the excess inductive reactance of a part of the transmission line disposed at the clearance hole area for which ground planes forming planar transmission lines are absent in the vertical direction. Especially, this problem is important if transverse dimensions of the clearance hole are large enough. This situation can be appeared when high-speed interconnections are designed because cross-sectional form and dimensions of the clearance hole are effective parameters to control the characteristic impedance.
In JP-A-2004-363975, a transition from a signal via pad to a coplanar transmission line interconnection is presented. In this consideration, a transition has also a form of coplanar transmission line having width of the signal strip and distance to the ground planes providing its characteristic impedance match to the coplanar transmission line interconnection. However, such transition is only formed for the coplanar transmission line at same conductor of the substrate and is applied for the square or rectangular clearance hole, that is, for the clearance hole in which the coplanar transmission segment can be formed. Also the width of the strip and distance between strip and ground plates at the same conductor layer must be chosen to provide predetermined characteristic impedance. In some cases, edge coupling between the signal strip of the transition and the coplanar transmission line interconnection in the place of the connection can increase their impedance mismatching.
In JP-A-H8-250912, a taper is used to connect a signal via and stripline. However, the length of the taper is small compared with the characteristic dimension of the clearance hole in the direction of the stripline and cannot effect on the excess reactance of the strip in the area of the clearance hole.